There are two basic types of gear couplings used for connecting rotating shafts in end-to-end relationship to transmit torque between two machines such as a drive motor and driven equipment. One type consists basically of gear hubs for the two shafts and two sleeves with internal gears meshing with the external gears on the hub for operationally connecting the hubs, and hence the shafts, to transmit the torque between the shafts. In this conventional type of gear coupling, the sleeves are provided with radially extending annular flanges at their center edges secured together by a plurality of bolts or screws spaced around the flanges. While this type of gear coupling is provided with a gear pitch diameter substantially larger than the normal diameter of the hub, and hence has a relatively large lubrication reservoir for either grease or oil, the bore capacity of the hubs is somewhat limited and the external diameter of the coupling is relatively large, thus limiting the applications of this type of coupling to installations in which the lateral space and centrifugal force are not critical limitations. In another basic type, which is essentially a modified version of the first one described, the hub diameter is larger, and hence provides a significant bore capacity; however, the capacity of the lubrication reservoir is relatively small and the seals are generally unreliable so that only grease lubrication can be safely used in the operation of the coupling. This type has the same type of radial flanges on the sleeves, and hence the same disadvantages as the first coupling with respect to size and centrifugal force. Notwithstanding the foregoing disadvantages of the two types of gear couplings described, these gear couplings have been extensively used in the mechanical power transmission field, since they have a relatively short axial length and require a short extension of the shaft beyond the machinery housing, thus minimizing shaft deflection and the load on the shaft bearings. Further, these types of gear couplings facilitate installation of the machinery and couplings by permitting the machinery to be installed in operating position relative to one another, with the shafts in proper alignment, and then the coupling connected without disturbing the precise operational relationship of the driving and driven machinery.
Another type of gear coupling has hubs similar to the hubs of the second type described above, but instead of using sleeves with radial flanges to interconnect the hubs, a single cylindrical sleeve is mounted on the hubs and internal gearing near each end meshes with the external gearing on the two hubs. While this type of gear coupling reduces the external diameter of the coupling, and hence reduces the centrifugal force as compared with the flange type coupling, the sleeve is relatively long axially and may prevent installation of the coupling after the machinery has been installed in final operating position. Further, the reservoir for lubrication is normally restricted, thus requiring frequent servicing to prevent premature failure of the coupling. However, the service life of this type and the flange type coupling is normally approximately the same when proper lubrication is maintained.